1. Field of the Invention
The present invention concerns an optical switch of the kind which has an optical input and two optical outputs and receives an electrical control signal. It couples the input to one or other of the outputs, the coupled output being selected by the electrical signal.
2. Description of the Prior Art
Switches of this kind are used, for example, in optical telecommunication networks to constitute switching matrices. It is desirable for these matrices to have high capacities and to be compatible with wavelength division multiplexing. An optical switch for this purpose must have a wide bandwidth, a low sensitivity to polarization, low insertion losses and, most importantly, a high extinction rate. The extinction rate is the ratio of the luminous power at the selected output to that at the other output.
Directional optical couplers are known and have a high extinction rate. They are sensitive to polarization and have a narrow bandwidth, however.
Y-branch digital optical switches (DOS) are also known and have a generic structure described in detail below.
They are insensitive to polarization. Modifications have been made to them to increase their extinction rate. They are described, for example, in an article in Journal of Lightwave Technology, Vol. 11, No. 2, February 1993, pp 379-387, "Reduction of Voltage-length Product for Y-Branch Digital Optical Switch", H. Okayama and M. Kawahara.
The extinction rate of these switches can be increased by increasing the power of the control signal. Any such increase is costly, however, and can cause problems with the removal of heat, especially in the case of a high-capacity switching matrix. This is why the extinction rate of known Y-branch digital optical switches in practise remains too low for the expressed requirements.
One object of the present invention is to increase the extinction rate of a Y-branch digital optical switch in a simple manner and without increasing the power of a control signal. Another object of the present invention is by this means to provide optical switching matrices of increased capacity.